Hydration of ions in dipolar aprotic solvents

Abstract

Interactions between water at low concentrations and 1:l electrolytes have been studied in four dipolar aprotic solvents of increasing basicity: sulfolane (TMS) < acetonitrile (AN) < propylene carbonate (PC) << dimethyl sulfoxide (DMSO). The experimental methods used involved solubility, proton nmr, vapor pressure, and calorimetric determinations. Results are interpreted in terms of hydration constants K1 and enthalpies Ail1 for the 1:l complexes with CI-, NO3-, H+, Li+, Na+, and Ag+ ions in most of the cited solvents. The K1 values are somewhat insensitive to the nature of the ion, except for H+, but sensitive to the nature of the solvent. For DMSO all Kl values are particularly low, thus accounting for this solvent retaining its dipolar aprotic character in the presence of some water. The values of K1 (CI-) are directly proportional to Henry's law constants for water in each solvent. The enthalpy changes AH1 are low and nearly equal for the replacement of one AN or PC molecule by one water molecule in the solvation shells of CI-and Li+. It follows that the strong solvation of CI-by bulk water is due to cooperative effects. Comparison of the AH1 values with gas-phase ionic hydration enthalpies shows contrasting behavior for negative and positive ions. The calculated values of the solvation enthalpies of the hydrated ions are discussed. Some comments are made regarding the acidity of H@+ in solvents. The comparison of rates or equilibrium constants of ionic reactions in water with those in dipolar aprotic (DPA) solvents has received a great deal of attention.] The observed solvent effects have been discussed in terms of changes in ionic solvation energies. Relatedly, the behavior of binary mixtures of water-DPA solvents offers further interest; a first step in understanding ionic solvation in these complex media is to study water-poor binary mixtures. A knowledge of ionic hydration in such aqueous media also throws light on the role of water as a solvent, since the self-association of water in DPA solvents may be reduced or eliminated according to its concentration and the basicity of the ~o l v e n t .~ It can then be advanced that ionic hydration in aqueous DPA solvents bears some relation to the now well-documented hydration of ions in the gas phase.4The present work was undertaken to study the interactions between water at low concentrations and 1:l electrolytes, mainly perchlorates and tetraethylammonium salts, in four DPA solvents, in order to provide thermodynamic data for a comparison of the hydration of C1-, NO3-, H+, Li+, Na+, and Ag+ in these aqueous media. The ions NO3-, Li+, Na+, and Ag+ were selected to afford an eventual comparison with hydration in molten nitrates and concentrated nitrate aqueous solutions. The DPA solvents chosen, sulfolane (TMS), acetonitrile (AN), propylene carbonate (PC), and dimethyl sulfoxide (DMSO) have different donor groups and increasing basicities in the order T M S < AN < PC << DMSO. The experimental methods used to obtain the 1:l ionic hydration constants or to redete...

Molecular dynamics study of hydrated alkali and halide ions in liquid nitrobenzene

“…For example, in phase transfer catalysis, water molecules may accompany the transfer of an ionic reactant from water to the organic phase and thus significantly affect its reactivity [1]. Several experiments such as UV-vis [2], proton NMR [3][4][5][6], and IR [7] have confirmed that some water molecules are coextracted when reagents such as tetraphenylborate are used to extract hydrophilic inorganic ions from water to nitrobenzene. Accurate numbers of the co-extracted water molecules per ion have been established in nitrobenzene [8,9].…”

Hydration of Some Acids in Sulfolane

“…Having established that monomeric species predominate for both acid and water (at low water concentrations), we of Huyskens and co-worker (20), in acetonitrile, were able to interpret our vapor pressure results suggest that H 2 0 behaves as a H-bond acceptor in terms of the association reaction weaker than C1-and also Br-. The low "basicity" of isolated H 2 0 molecules compared with the high basicity of bulk water has already The water vapor pressure curves shown in been remarked upon (22,6). Our observed low Fig.…”

“…Our hydration constants K, for methanesulfonic, picric, and 3,5-dinitrobenzoic acids in Sulfolane are not too different from, but somewhat lower than the same constants K, in acetonitrile, which are respectively 14, 0.5, and 1.8 (3). This difference may result from slightly weaker interactions of the acids with acetonitrile than with Sulfolane since water interactions with both solvents are similar (6). However, in a more recent study Kolthoff and Chantooni concluded from near infrared measurements that picric acid is not hydrated in acetonitrile (7).…”

Thermodynamic study of hydrogen bonding to chloride ions

“…The treatment for obtaining values of the 1:1 complex formation constant K and complex shift S¡ is essentially as described earlier. 9 Our results for sulfolane together with values for AN are given in Table I. The highest values of K are somewhat ap- proximate since the curvature of the plots ofvs.…”